The present invention relates to U.V. laser surface treatment methods, particularly to the removal of foreign materials from substrate surfaces. More particularly, the invention relates to an improved chemistry for dry stripping process employing UV-laser radiation and reactive chemistry for the removal of photoresist from semiconductor wafers.
In the manufacturing of various products it is necessary to apply a layer of protective material on a surface, which must be removed after a specified manufacturing step has been concluded. An example of such process is the so-called xe2x80x9cphotolithographyxe2x80x9d process, which is widely used in the manufacturing of integrated circuits. In this process, a pattern is created on a surface using a layer of protective material illuminated through a mask, and the surface is then treated with a developer which removes material from the unmasked portions of the surface, therefore leaving a predetermined pattern. The surface is then treated by ion implantation or by etching agents, which introduce the implanted species into the unmasked portions of the surface, or removes material from unmasked portions. Once these processes are completed, the role of the protecting mask ends and it must be removed. The process is conventional and well known in the art, and is described, e.g., in R. K. Watts, xe2x80x9cLithographyxe2x80x9d, VLSI Technology, S. M. Sze, ed., McGraw-Hill, N.Y., 1988, Chpt. 4.
Two main photoresist stripping methods exist in the modern VLSI/ULSI (Very/Ultra Large Scale Integration) circuits industry (D. L. Flamm, xe2x80x9cDry PlasmaResist Strippingxe2x80x9d, Parts 1, 2 and 3; Solid State Technology, August, September and October 1992):
1) Wet stripping which uses acids or organic solvents;
2) Dry stripping, which uses plasma, O3, O3/N2O or U.V./O3-based stripping.
Both methods are problematic and far from being complete, especially when taking into consideration the future miniaturization in the VLSI/ULSI industry. The current technology is capable of dealing with devices having feature sizes of about 0.5 xcexcm, but before the end of the century the expectation is that the workable size of the devices is expected to be reduced to 0.25 xcexcm. The expected size changes require considerable changes in the manufacturing technology, particularly in the stripping stage. The prior art photoresist stripping techniques described above will be unsuitable for future devices, as explained hereinafter.
Utilizing only the wet stripping method is not a perfect solution, as it cannot completely strip photoresist after tough processes that change the chemical and physical properties of the photoresist in a way that it makes its removal very difficult. Such processes include, e.g., High Dose Implantation (HDI), reactive Ion Etching (RIE), deep U.V. curing and high temperatures post-bake. After HDI or RIE the side walls of the implanted patterns or of the etched walls are the most difficult to remove.
In addition, the wet method has some other problems: the strength of stripping solution changes with time, the accumulated contamination in solution can be a source of particles which adversely affect the performance of the wafer, the corrosive and toxic content of stripping chemicals imposes high handling and disposal costs, and liquid phase surface tension and mass transport tend to make photoresist removal uneven and difficult.
The dry method also suffers from some major drawbacks, especially from metallic and particulate contamination, damage due to plasma: charges, currents, electric fields and plasma-induced U.V. radiation, as well as temperature-induced damage, and, last but not least, from incomplete removal. During various fabrication stages, as discussed above, the photoresist undergoes chemical and physical changes which harden it, and this makes the stripping processes of the prior art extremely difficult to carry out. Usually a plurality of sequential steps, involving wet and dry processes are needed to remove completely the photoresist.
The art has addressed this problem in many ways, and commercial photoresist dry removal apparatus is available, which uses different technologies. For instance, UV ozone ashers are sold, e.g. by Hitachi, Japan (UA-3150A), dry chemical ashers are also available, e.g., by Fusion Semiconductor Systems, U.S.A., which utilize nitrous oxide and ozone to remove the photoresist by chemical ashing at elevated temperatures, microwave plasma ashing is also effected, e.g., as in the UNA-200 Asher (ULVAC Japan Ltd.). Also plasma photoresist removal is employed and is commercially available, e.g., as in the Aspen apparatus (Mattson Technology, U.S.A.), and in the AURA 200 (GASONICS IPC, U.S.A.).
More recently, photoresist removal has been achieved by ablation, using laser UV radiation, in an oxydizing environment, as described in U.S. Pat. No. 5,114,834. The ablation process is caused due to strong absorption of the laser pulse energy by the photoresist. The process is a localized ejection of the photoresist layer to the ambient gas, associated with a blast wave due to chemical bonds breaking in the photoresist and instant heating. The partly gasified and partly fragmented photoresist is blown upwards away from the surface, and instantly heats the ambient gas. Fast combustion of the ablation products occurs, due to the blast wave and may also be due to the photochemical reaction of the UV laser radiation and the process gases. The main essence of the process is laser ablation with combustion of the ablated photoresist which occurs in a reactive gas flowing through an irradiation zone. The combination of laser radiation and fast combustion provides simultaneous lowering of the ablation threshold of hard parts of the photoresist (side walls). The combusted ablation products are then removed by vacuum suction, or by gas sweeping leaving a completely clean surface.
While U.S. Pat. No. 5,114,834 provides an important novel process, it still does not provide a high throughput, which is industrially convenient, viz., an industrially acceptable number of wafers that can be stripped during a given time. The laser stripping throughput is determined by the stripping rate or by the number of laser pulses needed for providing complete stripping of a unit area of the photoresist per unit of time.
While reference will be made throughout this specification to the ablation of photoresist from semiconductor wafers, this will be done for the sake of simplicity, and because it represents a well known and widely approached problem. It should be understood, however, that the invention described hereinafter is by no means limited to the stripping of photoresist from wafers, but it applies, mutatis mutandis, to many other applications, such as stripping and cleaning of photoresist from Flat Panel Displays (FPD) or removal of residues from different objects, such as lenses, semiconductor wafers, or photomasks.
It has now surprisingly been found, and this is an object of the invention, that it is possible to accelerate a given laser removal process by reducing the number of pulses while carrying out the stripping process in the presence of a specific gas composition consisting of O2/O3 and NxOy, wherein NxOy indicates one or more nitrogen oxides, x and y having the appropriate values for the given oxide or mixtures of oxides.
Four different gas compositions are preferred according to the present invention:
1) O2:O3 (Comparative)
2) O2/O3:NxOy 
3) O2:NxOy 
4) NxOy 
This surprising result is obtained by providing additive amounts of NxOy gases in the reactive gas stream which oxidizes the ablated photoresist and enhances laser etching of hard side-walls. The effect of NxOy is significant, since contents of less than 5% of the total gas flow are capable of doubling the stripping throughput. Gas percentages given herein are given by volume, unless otherwise specified.
The method of improving the throughput of stripped substrates in an laser stripping process, according to the invention, comprises adding to the reactive gas mixture an accelerating effective amount of one or more gases of the formula NxOy, wherein x and y have the appropriate values for the given oxide or mixture of oxides, to reduce the number of pulses needed for a complete removal. Typically, NxOy comprises a gas selected from the group consisting of NO, N2, N2O3, N2O4, NO3 and NO4.
As will be appreciated by the skilled person, NxOy can be generated by a number of methods. According to a preferred embodiment of the invention, NxOy is generated by exciting N2O gas. A simple and effective method of exciting N2O gas is by silent or corona discharge.
The process is schematically illustrated in FIG. 1. Oxygen is excited in a first corona discharge ozone generator indicated by numeral 1, to produce a mixture of O2 and O3, in proportions depending on the discharged energy in the generator. N2O is excited in a second corona discharge device indicated by numeral 2, to produce a mixture of NO, NO2 and N2O gases (NxOy). The NxOy, O2 and O3 are fed to the process chamber (numeral 3). In the process chamber the wafer which is swept by the aforementioned reactive gases is irradiated by U.V. Laser radiation 4, and stripping takes place. The gases passing through the chamber are exhausted through the vent 5.
As stated, even very low amounts of NxOy are useful in effectively reducing the number of pulses needed to completely remove the film. According to a preferred embodiment of the invention the content of NxOy in the total reactive gas stream varies according to the pressure in the process chamber. Typical pressures are in the range of 0.1-2 bar, preferably 0.3-0.99 bar. For example, at xcx9c0.4 bar the optimum content of excited N2O is xcx9c5% (v/v) (See FIG. 2). As will be appreciated by the skilled person, the optimal content of NxOy will vary as a function of many operating conditions, such as the laser pulse energy fluence, the pulse repetition rate, the laser wavelength, the laser beam incidence angle, the height of the optical window above the wafer surface, the total pressure in the process chamber, the substrate temperature, the scanning velocity of the laser beam footprint, the flow velocity of the reactive gas mixture, as well as the partial pressures of reactive gas mixture components. These parameters will have to be determined specifically for specific stripping tasks. Determination of the optimal O2/O3:NxOy ratio, however, can be easily carried out by the skilled person for a given system, and does not impose an excessive burden on the planning of operating conditions. The optimal operating conditions may be determined, for example, by using statistical optimization software.
Without wishing to be bound by any particular theory, the inventors believe that the surprising results achieved by the invention are due to the processes illustrated in the xe2x80x9cReaction Schemexe2x80x9d in FIG. 3.
Other gases may be added to the O2/O3:NxOy gas composition to improve the removal, especially of inorganic residues. Such gases must include chlorine and/or fluorine atoms, e.g., Cl2, F2, NF3, NOCl, NOF, Cl2O, ClO2, CF4, CClF3, HCl, HF, SF4 and SF6.
The invention also encompasses the apparatus for carrying out the laser stripping of a coated or contaminated surface according to the invention. Such apparatus comprises:
a laser U.V. radiation source;
a process chamber suitable to contain the surface to be stripped, said chamber being provided with gas inlet(s) and gas outlet(s);
at least one source of oxygen or of ozone; and
at least one source of NxOy.
The above and other characteristics and advantages of the invention will be better understood through the following illustrative and non-limitative examples.